1. Field of the Invention
The present invention relates to a Group III nitride semiconductor light-emitting device including an epitaxially grown layer which has been removed from a growth substrate through the lift-off process and is bonded to a support. The present invention also relates to a light-emitting device including a support formed of a ceramic substrate.
2. Background Art
In general, a sapphire substrate has been used as a growth substrate for a Group III nitride semiconductor. However, sapphire poses problems in terms of electrical conductivity and thermal conductivity. For example, a Group III nitride semiconductor light-emitting device having a device structure formed on a sapphire substrate exhibits poor linearity in a high-current region or poor durability upon long-term operation. Sapphire has no cleavage property and exhibits high physical and chemical strength. Therefore, when a light-emitting device is produced from a sapphire substrate, separation of the device into chips requires an intricate process (e.g., a process of thinning the sapphire substrate), which causes an increase in production cost. Another cause of high production cost is that a sapphire substrate itself is expensive.
In order to solve such problems, attempts have been made to develop a technique in which a Group III nitride semiconductor is grown on a growth substrate, and subsequently the growth substrate is removed therefrom (i.e., substrate lift-off process).
One such technique is the laser lift-off process. In this process, after bonding of a Group III nitride semiconductor layer to a support substrate, the interface between a growth substrate and the Group III nitride semiconductor layer is irradiated with a laser beam, to thereby decompose the Group III nitride semiconductor layer for separation and removal of the growth substrate. Meanwhile, a so-called chemical lift-off process has been developed as a technique for removal of a growth substrate. In this process, a chemical-soluble layer is incorporated into a Group III nitride semiconductor layered structure as a layer that is nearest to a growth substrate, and, after bonding of the Group III nitride semiconductor layered structure to a support substrate, the chemical-soluble layer is dissolved by a specific chemical, to thereby remove the growth substrate.
Japanese Patent Application Laid-Open (kokai) No. 2008-205005 discloses a technique for improving the light extraction performance of a Group III nitride semiconductor light-emitting device from which a growth substrate has been removed and which is bonded to a support substrate. In the light-emitting device described in Japanese Patent Application Laid-Open (kokai) No. 2008-205005, a dent for light reflection is provided so as to extend from the surface of a p-type layer (on the side where the p-type layer is bonded to a p-electrode) to an n-type layer, and side walls of the dent are inclined such that the area of a horizontal cross section of the dent parallel to the main surface of the device is reduced with decreasing distance between the cross section and the n-type layer. By means of this dent for light reflection, light which is confined in the vicinity of an active layer and is propagated in directions parallel to the main surface of the device is reflected onto the side of the n-type layer, whereby light extraction performance is improved.
Japanese Patent Application Laid-Open (kokai) No. 2009-176966 discloses a semiconductor device produced through the laser lift-off process, which includes a support substrate formed by providing electrode layers on both surfaces of an AlN or SiC ceramic substrate, and electrically connecting the electrode layers by means of vias. This patent document proposes employment of such a support substrate for solving a problem in terms of thermal conductivity and for achieving electrical conduction in a direction perpendicular to the main surface of the substrate. This patent document also proposes employment, as a growth substrate for a Group III nitride semiconductor, of an Si substrate which has a large size and is less expensive than a sapphire substrate.
However, even when a dent for light reflection is provided as described in Japanese Patent Application Laid-Open (kokai) No. 2008-205005, a portion of light reflected by the dent onto the side of an n-type layer is reflected onto the surface of the n-type layer (on the side where the n-type layer is bonded to an n-electrode) and returns to the interior of the device. Thus, light extraction performance fails to be improved sufficiently.
Conceivably, light extraction performance may be improved by reducing the area of the n-electrode formed on the surface of the n-type layer. However, in such a case, current is not diffused sufficiently in directions parallel to the main surface of the device, and uniformity of emitted light is impaired. In addition, light emission performance is considerably reduced upon operation at high current density.
The method described in Japanese Patent Application Laid-Open (kokai) No. 2009-176966 can solve a problem in terms of processability and thermal conductivity of a sapphire substrate. However, this method cannot solve a problem in terms of an increase in production cost, since a sapphire substrate is generally used as a growth substrate. In addition, when a ceramic substrate is used as a support substrate as described in Japanese Patent Application Laid-Open (kokai) No. 2009-176966, the ceramic substrate must be processed so that electrical conduction is achieved in a thickness direction, and thus production cost increases with increasing number of production processes. Other causes of an increase in production cost are that the laser lift-off apparatus employed is expensive, and production yield is low.
meanwhile, a Group III nitride semiconductor light-emitting device having a device structure formed on an Si substrate realizes reduction in production cost, since the device does not employ an expensive sapphire substrate. However, such a light-emitting device poses a problem in that light extraction performance is low due to absorption of light by the Si substrate.